Kevin Durant heated up in the fourth quarter to score 31, and Serge Ibaka added 18 points and 16 rebounds for the Thunder.

But it was Westbrook's 17-foot pull-up jumper with one minute left that broke a 101-101 tie and ultimately proved to be the game-winner.

Northwest Division-leading Oklahoma City improved to 50-17 -- the franchise's third 50-win season since relocating from Seattle. The Thunder are one game behind San Antonio (51-16) in the race for the top seed in the Western Conference. Oklahoma City has already clinched a playoff berth.

Dallas (31-35) continues to cling to its slim postseason hopes. The Mavericks came into the game three games back of the eighth-place Los Angeles Lakers.

Oklahoma City has now beaten the Mavericks 10 consecutive times, including last season's four-game first-round playoff sweep. This one, like most games in the rivalry, was close, but Oklahoma City continues to show how far it has come since losing to Dallas in the 2011 West finals.

"Every time we play Dallas, it seems that we should start the game with a two-minute game at 0-0," Thunder coach Scott Brooks said. "That's what it seems to come down to every game. It comes down to the last two or three minutes of the game.

"I like the way we competed, and we showed a competitive spirit throughout the game. We had a run, they had a run, we had a run, it kept going back and forth and neither team gave up. I'm just glad that we had the last run."

Dirk Nowitzki made his first eight shots on his way to a team-high 23 points, but faded down the stretch. Vince Carter scored 18 off the bench for Dallas, which also had Mike James, Darren Collison and Jae Crowder score in double figures.

Guarded mainly by Kendrick Perkins down the stretch, Nowitzki didn't register an official shot attempt in the fourth quarter.

"We didn't do a good job. I take responsibility for it," Mavs coach Rick Carlisle said. "A few times they fouled him before he could get a shot off. He got to the free throw line two or three times.

"When they get into all of that switching and denying, sometimes other guys have got to step up and make plays, and we weren't able to make plays."

The Thunder's one-point lead at halftime stood up going into the final period. Oklahoma City went on a 7-0 run starting with the last basket of the third quarter and the first five points of the fourth. Dallas responded with its own 7-0 run to take a one-point (79-78) lead.

"They just made more plays down the stretch," Dallas forward Elton Brand said. "We had Durant under control for about 75 percent of the game. He just came up big and hit some big shots. Westbrook hit a big shot. They scored and we didn't. We fought well and we were right there."

Oklahoma City would go up 88-81, but James (14 points) knocked down two 3-pointers as Dallas closed the gap to 92-91. Durant heated up by scoring 12 of the Thunder's 14 next points to give the visitors a temporary cushion.

"They were obviously playing for their playoff lives and they gave great energy," Brooks said. "Unfortunately, somebody had to lose. You had a good NBA basketball game. Both teams can be happy with the way they competed. It was a classic example of that tonight. KD had an amazing fourth quarter and he took over that game, and that's what he can do."

Neither team could separate much in the first half, as the Thunder took a 50-49 into the break. Westbrook racked up 16 points by halftime, with Durant adding 12.

Nowitzki, who was 5-for-5 from the floor in the first half, and Carter each scored 11.

Dallas came out strong, especially James, in taking an early 13-8 lead. The Mavericks led by as many as six and took a 26-24 lead into the second quarter. Oklahoma City remained close because of Westbrook, who scored 14 in the first period on 5-of-7 shooting.